Digital Signal Processing (DSP) techniques enable to compensate the impairments affecting optical signals during transmission in an optical fiber by applying the inverse filter properties of the impairments. This enables the transmission of higher rate channels on longer reach. These techniques can be applied at a receiver receiving a light wave signal through a medium such as an optical fiber. Notably, coherent reception enables to get the information on both phase and amplitude of the received signal. In this way, the DSP compensates the impairments occurring during transmission, before reception, by using digital filters. Signal equalization is realized with a DSP implemented in a signal processor. Besides, using digital signal processing enables the demodulation of multilevel signals such as Quadrature Amplitude Modulation (QAM) signals, which enables to increase the capacity of an optical fiber link. The reception of 16QAM signal is realized with digital processing in the receiver.
However, the benefits of the digital processing are not limited to the application of this technique at the receiver end. DSP techniques combined with Digital to Analog Converter (DAC) can be applied at the transmitter side. In such a case, the transmitter, which is called a digital transmitter hereafter, includes a DSP processor and a DAC to convert digital signals into analog signals used to drive an IQ modulator.
In such a manner, a DSP of a digital transmitter can be used to pre-compensate at the transmitter side for linear impairments appearing during transmission in the fiber such as chromatic dispersion (CD). Independently, a transmitter can be used to generate complex modulation formats such as but not limited to QAM signals.
The configuration and the setting of the transmitter can be changed at startup, during the operation to accommodate optimized reach and bandwidth allocation in the network, or in changing transmission routes inside dynamic networks. An example of a reconfigurable digital transmitter which is called a software defined transmitter and is capable of emitting signals chosen from eight different formats is described in the non patent literature 1 (NPL1). Here, reconfigurable transmitters require changing their transmission formats at high speed. Namely, the data loss at the configuration changing must be avoided because the transmission link would not be operable in such a case. For instance, the changes in the characteristics of the transmission line, which are caused by external elements, degradation of equipment or working conditions, or path reconfiguration, would trigger the reconfiguration of modulation format or pre-distortion configuration. This can be done with the reconfigurable transmitter described in NPL1, for example. However, such a reconfiguration must be realized without data loss.
The modulation in a digital transmitter can be performed with an optical IQ modulator, which is sometimes called Cartesian modulator, vector modulator, Dual Parallel modulator, or nested modulator depending on the sources. In an IQ modulator, the electric signals drive two independent Mach-Zehnder devices, which can be called children Mach-Zehnder Modulators (MZMs). The children MZMs modulate the phase and amplitude of the same optical carrier and their outputs are relatively phase delayed by 90 degrees before being recombined. These components are called In Phase (I) and Quadrature Phase (Q) of the signal. The phase difference between the outputs of the children MZMs can be called the angle of quadrature and is equal to 90 degrees ideally. Such an IQ modulator is used in the transmitter described in NPL1. In the case of a reconfigurable transmitter illustrated in NPL1 for example, the driving signal of the IQ modulator is changed according to the set modulation format or the pre-distortion configuration.
However, it is known that there is a drift of the DC (Direct Current) bias in the IQ modulator due to the variation of the temperature or the ageing of the device. There are three types of affected biases, that is, the DC bias of each of the two children MZMs and a DC bias used to set the angle of quadrature. This causes a degradation of the transmitted signal, and therefore results in the degradation of the received signal quality or in worst cases the impossibility to decode the received signal. This problem is likely to be revealed in the characterization tests of the modulator at the production stage or at the assembly stage of the transmitter in which the modulator is used, and when it is used. This problem can be solved by using Auto Bias Control (ABC) circuits, which control the biases of the modulators and compensate the DC bias change. In this manner, the ABC technology can manage the drift of DC bias of the IQ modulator and enables a correct modulation in the optimal condition.
An example of an ABC circuit is illustrated in the non patent literature 2 (NPL2). The ABC circuit illustrated in NPL2 is based on low frequency dither tones to control the DC biases of the children MZMs of I and Q components as well as the angle of quadrature. The reported convergence time of the ABC circuit is 1 minute. It is sufficient to track variations of environmental temperatures or the degradation of the device. Making this convergence time faster would be possible, but the order of magnitude of the convergence time would not change because the ABC is based on a low frequency tone. Furthermore, very fast ABC tracking of DC biases, which change due to slow environmental temperature changes or slow device ageing, would result in an unstable feedback.